This invention generally relates to electrochemical cells utilizing an intercalation cathode. More particularly, this invention relates to electrochemical cells utilizing an intercalation cathode, a non-aqueous gel polymer electrolytic system and a magnesium anode.
Since the 1980s there have been attempts to develop rechargeable magnesium batteries. These batteries may be regarded as an analog of the well-known Li battery, because both of the batteries are based on the same intercalation phenomenon: Li+ and Mg2+ ions from the electrolyte solution are inserted into the host cathode material upon discharge of the battery and return to the solution (and to the anode) upon the charge of the battery. Rechargeable, high energy density electrochemical cells of various kinds are known. Such cells usually consist of a transition metal oxide or chalcogenide cathode-active material, an anode-active alkali metal or alkali metal intercalation compound, and an electrolyte solution containing a dissolved alkali-based salt in an aprotic organic or inorganic solvent, or polymer electrolyte.
Theoretically, a rechargeable cell is capable of charging and discharging indefinitely, however, in practice such performance is unattainable. The degradation mechanisms of the various anodes, cathodes and electrolytes are complex and are known to those skilled in the art.
Two basic types of cathodes are appropriate for a battery system that is rechargeable at ambient temperatures. A liquid cathode can be used, allowing reactions to take place with facility. Liquid cathodes are also advantageous in that thin films or crusts forming on the surface of the cathode tend to crack, such that the cathode activity remains high over the course of the cycling. The mobility of the cathodic material is a liability, however, in that contact with the anode short-circuits the cell. Thus, an electrochemical cell with a liquid cathode requires protective, insulating films on the anode.
A solid cathode must be insoluble in the electrolyte, and must be able to absorb and desorb a charge-compensating ion in a substantially reversible and fast manner. A prime example of a solid cathode of this variety is an intercalation cathode. Intercalation chemistry focuses on the insertion of ions or neutral molecules into an inorganic or organic matrix. In a typical intercalation cathode, cations dissolved in the electrolyte solution are inserted into the inorganic matrix structure.
In contrast to Li, Mg batteries should be relatively safe, cheap and green (without dangerous toxic materials). However, no material was found suitable for the Mg insertion. In spite of the evident similarity between Li and Mg intercalation, almost all inorganic compounds, which prove themselves in Li batteries, are not active or show extremely slow kinetics in the case of Mg. U.S. Pat. No. 6,316,141 teaches that only a group of intercalation materials of particular importance is called Chevrel-phase materials, also known as Chevrel compounds, allows a relatively fast insertion of Mg2+ ions owing to their unusual crystal structure. Chevrel compounds contain an invariant portion consisting of molybdenum and a chalcogen—sulfur, selenium, tellurium, or mixtures thereof. The invariant portion is generally of the formula Mo6Tn, where T represents the chalcogen and n is usually about 8. The unique crystal structure of Chevrel-phase materials allows the insertion of one or more metal ions in a reversible, partially reversible, or irreversible manner. The stoichiometry of the intercalation compound can be represented as MxMo6Tn, where M represents the intercalated metal, x may vary from 0 (no intercalated metal) to 4 or less, depending on the properties of the particular anion T (T is S, Se, Te).
The intercalation of metal ions into the Chevrel compound releases energy. Since the process is partially or fully reversible, these compounds are particularly suitable as electrodes in electrochemical cells. For example, lithium, the predominant intercalation ion, can be removed from the Chevrel compound by the application of electrical energy. The energy is released as electrical energy upon reintercalation.
The following two compositions are the most attractive as a cathode: MgXMo6S8 and a metal-free sulfide, Mo6S8, with a maximal theoretical capacity of 122 mA*h/g (the smallest molecular weight). However MgXMo6S8 obtained by direct high-temperature synthesis, is electrochemically inactive due to the insulating MgO film formed on the surface of the active material. While Mo6S8 shows a good electrochemical performance, an essential part (about 30%) of the initial capacity is lost in the first cycle. This loss results from the Mg trapping in MgXMo6S8 when X<1. Different ways to avoid the trapping and to improve the electrodes' kinetics were suggested such as an additional potentiostatic stage at high voltage upon battery charge and milling of the active mass. However, all these operations complicate the battery preparation and exploitation, and only solve the problem partially: 90-100 mA*h/g (75-80% of the theoretical capacity) can be obtained at 4.5 h of the battery discharge, but this value decreases essentially for higher current density because the trapping is associated with the relatively low kinetics of Mg insertion into MgXMo6S8, when X<1 (relatively low rate capability).
Thus, there is still a long felt need for ways to avoid the trapping of the Mg2+ ions, which caused by their motion in a circuitous manner within definite groups of the cation sites, instead of the progressive diffusion between different types of sites needed for the normal function of the electrode material.
Furthermore, the cathode-active material in the high energy density, rechargeable electrochemical cells must be paired with a suitable anode-active material, which is most commonly made of an active metal such as alkali metals. However, the performance of a particular anode-cathode couple is strongly influenced by the nature of the electrolyte system. Certain non-aqueous electrolytes are known to perform well with a particular anode-cathode couple and be ineffective or significantly less effective with other anode-cathode couples, either because the electrolyte solution's components are not stable or because the solutions components degrades during cycling active electrodes. As a result, much of the prior art relates to the cathode-active material, the anode-active material and the electrolyte not only as independent entities, but also as units within an appropriate battery system.
U.S. Pat. No. 4,104,451 to Klemann et al., discloses reversible batteries with an alkali metal anode, a chalcogenide cathode, and organometallic alkali metal salts in organic solvents as the electrolyte system. Non-aqueous electrolyte systems containing alkali metal salts of boron or aluminum anions based which also contain organic groups are disclosed.
Organoborate salts of alkali metals represented by the formula
are disclosed in U.S. Pat. No. 4,511,642 to Higashi et al., wherein R1-R4 are organic radicals selected from the following groups: alkyl, aryl, alkenyl, cycloalkyl, allyl, heterocyclic, and cyano, and M.sup.+ represents an alkali metal ion.
U.S. Pat. No. 4,139,681 describes cells containing electrolytically active metal salt complexes having the formula ZMR.sub.n X.sub.i, wherein Z is a metal from a group containing aluminum, the Rs are specified haloorganic radicals, the Xs are selected from various halides, alkyls, aryls, alkaryls and aralkyls. M is specified to be an alkali metal, with lithium being the preferred embodiment.
U.S. Pat. No. 4,542,081 to Armand et al., describes solutions for the constitution of solid electrolyte materials of electrochemical generators. The compound is of the formula(R—C≡C)4Z−,M+in which Z is a trivalent element capable of entering into 4-coordination, such as aluminum, and R represents groups which are non-proton donors. M is specified to be an alkali metal.
The prior art described above, including U.S. Pat. Nos. 4,104,451, 4,511,642, 4,139,681 and 4,542,081, specifies that M is an alkali metal. The use of an alkaline earth metal anode such as magnesium would appear disadvantageous relative to the use of an alkali metal such as lithium because alkali metal anodes are much more readily ionized than are alkaline earth metal anodes. In addition, on recharge the cell must be capable of re-depositing the anode metal that was dissolved during discharge, in a relatively pure state, and without the formation of deposits on the electrodes.
However, there are numerous disadvantages to alkali batteries. Alkali metals, and lithium in particular, are expensive. Alkali metals are highly reactive. Alkali metals are also highly flammable, and fire due to reaction of alkali metal with oxygen or other active material is extremely difficult to extinguish. Lithium is poisonous and compounds thereof are known for their severe physiological effects, even in minute quantities. As a result, the use of alkali metals requires specialized facilities, such as dry rooms, specialized equipment and specialized procedures.
In contradistinction, magnesium metal and aluminum metal are easy to process. The metals are reactive, but undergo rapid passivation of the surface, such that the metals exhibit highly stable behavior. Both magnesium and aluminum are inexpensive relative to the alkali metals.
U.S. Pat. No. 4,894,302 to Hoffman et al., discloses an electrochemical cell having an intercalation cathode, an alkaline earth anode, and a non-aqueous liquid electrolyte containing an organic solvent and an electrolytically active, organometallic alkaline earth metal salt represented by the formula
wherein Z is boron or aluminum; R1-R4 are radicals selected from the following groups: alkyl, aryl, alkaryl, aralkyl, alkenyl, cycloalkyl, allyl, heterocyclic alkyl, and cyano; and M represents an alkaline earth metal such as magnesium. The radicals can be inertly substituted with substituents that have no detrimental effect upon the electrolytic properties of the electrolyte composition with respect to effectiveness in an electrochemical cell, such as halogenated or partially halogenated derivatives of the above groups. While exhaustive care is taken to disclose a broad range of organic radicals and halogenated organic radicals, bonding the metallic species of the anion (Z) to another inorganic species is not considered.
U.S. Pat. No. 5,491,039 describes a solid, single-phase electrolyte containing a solid polymeric matrix and an organometallic ion salt represented by the formulaMC(ZRn)wherein Z is boron, aluminum or titanium; Rn are various substituted or unsubstituted organic radicals; M is lithium, sodium, potassium, or magnesium, c is 1 or 2, and n is an integer from 1 to 6. As in U.S. Pat. No. 4,894,302, a broad range of organic radicals including halogenated organic radicals is disclosed, but the bonding of the metallic species of the anion (Z) to another inorganic species is not reported. In all cases, metallic species Z is bonded to a carbon atom. More specifically, the bonding of the metallic species of the anion (Z) directly to a halogen is not disclosed. It must be emphasized that this is of particular significance in light of the fact that U.S. Pat. No. 5,491,039 teaches an extremely broad range of radicals that may be appropriate for attaching to the metallic species of the anion.
Both U.S. Pat. No. 5,491,039 and U.S. Pat. No. 4,894,302 disclose electrochemical cells having an alkaline earth anode such as magnesium. For commercial application, however, such magnesium batteries must be essentially rechargeable and must have a reasonable shelf life. Sustaining a voltage of 1.5 volts is problematic or impossible with the usual intercalation cathodes and electrolytes according to prior art. Magnesium batteries operating at 1.5 volts are particularly prone to electrolyte decomposition and to encrustation/passivation of both electrode surfaces.
Thus, there is a long felt need for a rechargeable magnesium battery, which would be more safe, clean, efficient and economical than rechargeable batteries known heretofore.